ARIA: math role is intended for math images not real math, so it should just be role="img"

ARIA role="math" states, "This is for sections that represent math,  
such as images and ASCII art, but are not in a formal mathematical  
language. Such images MUST be labeled…" Since real math can be  
described by "a formal mathematical language," this is just an image,  
nothing more. As such, we should ditch the "math" role and just use  
the "img" role for all instances of images.